Completions lie at the heart of deepwater production and constitute a large portion of the overall well cost. Great multidisciplinary effort is invested in designing them. This contrasts greatly with the production stage, where little information is available to detect problems, optimize the inflow and prevent expensive work-overs. Sand screen plugging, incomplete packing, development of “hot spots” in screens, destabilization of the annular pack, fines migration, near-wellbore damage, cross-flow, differential depletion, compartmentalization, compaction represent a typical list of challenges that are extremely difficult to decipher based on just several permanent pressure and temperature gauges. Many problems can be identified by production logging, but it is costly and not in real time. Permanent pressure and temperature sensors placed across the sandface can provide critical information for diagnosing the completion problems and the service industry is developing tools to make such sensing feasible in the future. However these new pressure and temperature data are unlikely to lead to unambiguous identification of the problems above because of multitude of parameters characterizing complex completions and reservoirs that remain unconstrained by our data.
For example, there is an issue of underperforming hydrocarbon production wells in the Gulf of Mexico. “Well performance” absorbs large-scale reservoir issues such as compartmentalization as well as changes in local well skin with time that further comprises of completion, perforations and near-wellbore effects. Therefore, multiple explanations can be given to the problem. Apparent compartmentalization and ubiquitous U-shaped boundaries can be one answer on a “reservoir” scale. Yet those boundaries are rarely confirmed by 4D seismic or other data. Shale draping is an alternative reservoir-scale scenario that can lead to well underperformance. Another wellbore-scale explanation suggests that well productivity declines with time due to loss of so called “kh” product where k and h are reservoir permeability and thickness correspondingly. The differential depletion model argues that this loss occurs mainly due to reduction in producing thickness although the exact mechanisms of flow impairment are still debated. Similarly, reduction in permeability is another alternative explanation, although the amount of this reduction (85-90%) is not consistent with laboratory measurements. Existing sparse data from wells can support any of these scenarios, confirming that the problem is under-constrained. In order to distinguish between these quite different scenarios, there is a need for more downhole data at various scales that can unambiguously characterize various components of the production system.
In the context of deepwater completions there is an additional emphasis on sand control because it is believed that managing produced sand is generally a costly and mostly unworkable solution for the Gulf of Mexico, although it may work well in other places where there is some grain-to-grain cementation present. The presence of sand control media between the reservoir and the wellbore introduces additional cost, complexity and requires proper management. The goal is a solution that is robust enough to control sand production for the life of the reservoir, avoiding impairment and the need for any intervention. To come up with a sand control system that is less prone to problems, the root cause of existing problems must first be understood. Once issues are fully understood, smart, on-demand intervention or remediation may become possible. The high cost of deepwater well devices (sand screens etc) and intervention can justify the presence of smart surveillance tools that would not be economical in other environments. In addition, the surveillance tools preferably need to last for the life of the reserve.